1. Field of the Invention
The present invention relates to ferroelectric thin film device that functions as a piezoelectric device, a nonvolatile ferroelectric memory device, a pyroelectric device, or the like, and more particularly to a technique for controlling the orientation of a ferroelectric thin film, and to a technique for improving the bottom electrode of this ferroelectric thin film device.
2. Description of the Related Art
Crystalline materials consisting of compound oxides that exhibit ferroelectricity, such as lead titanate zirconate, barium titanate, and lithium niobate, have numerous functions, including spontaneous polarization, a high dielectric constant, an electro-optical effect, a piezoelectric effect, and a pyroelectric effect, and as such are used in the development of a wide range of devices. For instance, the piezoelectric properties of these materials are utilized in capacitors in FRAM (Ferroelectric Random Access Memory), DRAM (Dynamic Random Access Memory), and the like, their pyroelectric properties are utilized in infrared linear array sensors, and their electro-optical effect is utilized in wave-guide type light modulators, so these materials can be used in many different fields. Ferroelectric thin film devices having these various functions are also called functional devices.
It is often the case with a ferroelectric thin film device such as this that the characteristics vary with the crystal orientation of the ferroelectric thin film. For example, most lead titanate-based ferroelectrics, which are a type of ferroelectric having a perovskite type crystal structure, have a tetragonal crystal structure, and have spontaneous polarization in the c axis direction. Accordingly, spontaneous polarization in the direction perpendicular to the substrate can be maximized by orienting the c axis to be perpendicular to the substrate (c axis orientation treatment), allowing the performance of a ferroelectric thin film device in which this characteristic is utilized to be utilized to full advantage.
For this reason, it is important to control the crystal orientation in the formation of a ferroelectric thin film, and particularly a lead titanate-based ferroelectric film having a perovskite type crystal structure. Furthermore, since the electrical characteristics of these ferroelectric thin films vary with the orientation of the crystal plane, an orientation treatment must be performed according to the intended application of the ferroelectric thin film device. For example, a (100) priority orientation is known to be preferable with a nonvolatile ferroelectric memory device that makes use of the polarization characteristics of a ferroelectric thin film, such as a FRAM. With a piezoelectric device that is utilized as an electromechanical transducer (actuator), in the case of DC drive, a (111) priority orientation is known to be preferred because of the piezoelectric constant d31 characteristics in the drive voltage region.
Except in the case of natural orientation, the crystal orientation of a ferroelectric thin film is affected by the crystal orientation of the bottom electrode or substrate that serves as the base in the formation of the ferroelectric thin film. Accordingly, proper selection of the material of the bottom electrode or substrate that serves as the base is absolutely essential to controlling the orientation of a ferroelectric thin film. Generally, a silicon substrate is used as the substrate of a ferroelectric thin film, and a silicon dioxide film is formed in order to ensure good electrical insulation between the bottom electrode and the substrate, so a required characteristic of the bottom electrode is that it have good orientability even when formed on an amorphous film. Platinum electrodes have been used in the past as electrodes that satisfy this requirement. The lattice constant of a platinum electrode is matched to that of lead titanate zirconate, and because platinum is resistant to oxidation, no platinum oxide layer is formed at the interface with the dielectric layer, so the performance of the device tends not to deteriorate.
As to technology related to the bottom electrode, it has been reported in Japanese Patent Laid-Open No. 07-245236 that a structure having an iridium layer or an alloy layer of platinum and iridium as the bottom electrode is favorable in terms of the matching of the lattice constants of the bottom electrode and PZT. Japanese Patent Laid-Open No. 08-335676 deals with an improvement on this technology, reporting that if nuclei of a component element of PZT (titanium) are formed on the bottom electrode in a structure having an iridium layer or an alloy layer of platinum and iridium as the bottom electrode, crystals will grow around the nuclei and good contact with the PZT film can be ensured.
As to technology related to the substrate that serves as a base, it has been reported in Japanese Patent Laid-Open No. 5-281500 that a lithium niobate thin film is formed by sol-gel method on a sapphire (001) plane monocrystalline substrate. According to this technique, the axis of crystallization of a ferroelectric thin film can be uniaxially oriented by utilization of the crystallinity of the substrate.
However, even though it was possible to form a ferroelectric thin film with excellent orientability by optimizing the conditions that affect the orientation of a ferroelectric thin film by means of the bottom electrode (or substrate), such as matching the lattice constants of the bottom electrode (or substrate) and the ferroelectric thin film, as with the above-mentioned prior art, it was difficult to control the orientation of a ferroelectric thin film as desired according to the intended application of the ferroelectric thin film. For instance, if an attempt was made to vary the film formation conditions in the formation of a PZT film by sol-gel method, it was difficult to control the (100) priority orientation, which is favorable for a nonvolatile ferroelectric memory device, and the (111) priority orientation, which is favorable for the DC drive of an electromechanical transducer, as desired.
Also, diligent study by the inventors revealed that when a ferroelectric thin film device is used as an electromechanical transducer, the piezoelectric constant d31 will be higher if the ferroelectric thin film is set to a priority orientation of (111) in a drive frequency band of just a few kHz (low frequency band), and the piezoelectric constant d31 will be higher if the ferroelectric thin film is set to a priority orientation of (100) in a drive frequency band of several dozen kHz (high frequency band). This seems to be because the piezoelectric constant d31 remains more or less constant regardless of the drive frequency if the ferroelectric thin film is set to a priority orientation of (100), whereas the piezoelectric constant d31 decreases in value as the drive frequency goes up if the ferroelectric thin film is set to a priority orientation of (111). It is therefore desirable to be able to control as desired the orientation of a ferroelectric thin film according to the drive frequency of the electromechanical transducer.
Also, the technology disclosed in Japanese Patent Laid-Open No. 08-335676 allows crystals to be grown around nuclei and good contact with a PTZ film ensured by forming nuclei of a component element of PZT (titanium) on the bottom electrode, but if iridium alone was used as the bottom electrode and the PZT film was formed by sol-gel method, then there was a problem in that the bottom electrode took in oxygen and swelled in the course of the baking of the PZT film. Because the bottom electrode became hard and brittle if it took in oxygen, the bottom electrode would break if used as an actuator.
The structure that used to be employed when a ferroelectric thin film device was used as an electromechanical transducer had an adhesive layer (buffer layer) of titanium, chromium, or the like provided between the bottom electrode and the surface where this transducer was installed in order to enhance the adhesion between the electromechanical transducer and this installation surface. The inventors of the present invention, however, discovered that in the course of the manufacture of an electromechanical transducer, the element that makes up the adhesive layer, such as titanium, is diffused as a result of heat treatment and moves into the ferroelectric thin film, which diminishes the piezoelectric characteristics of the electromechanical transducer. The reason for this seems to be that the titanium becomes admixed with the ferroelectric thin film and disrupts the stoichiometric ratio in this film, or produces a layer with a low dielectric constant at the interface between the bottom electrode and the ferroelectric thin film.
A ferroelectric thin film has spontaneous polarization, and because the polarization direction can be inverted by the action of an external electrical field, this characteristic can be utilized to manufacture a nonvolatile memory. When a ferroelectric thin film was applied as a memory device, an alloy of platinum and iridium, iridium alone, or iridium oxide was used in the past as a bottom electrode for applying an electrical field to the ferroelectric thin film in an effort to enhance the characteristics of the ferroelectric thin film and prevent their deterioration over time. Examples of such usage are found in U.S. Pat. No. 5,191,510, Japanese Patent Laid-Open No. 07-245287, and elsewhere.
However, the bottom electrodes composed of an alloy of platinum and iridium, iridium alone, or iridium oxide that were used for memory devices posed problems when used as electromechanical transducers, such as in an ink jet recording head. Specifically, the electromechanical transducer must itself be deformed with an ink jet recording head, but with a bottom electrode containing iridium, there were problems in that the bottom electrode was too hard and the film stress generated in the bottom electrode was too high. Another problem was poor adhesion between the bottom electrode and the installation surface and between the bottom electrode and the ferroelectric thin film.
Furthermore, the problem of markedly increased leakage current was encountered when iridium was used as the bottom electrode, as discussed in the article “Explanation of the Leakage Mechanism of a PZT Capacitor Deposited on an Ir Electrode (Lecture Summaries from the 59th Convention of the Applied Physics Society, issued Sep. 15, 1998, p. 450). Accordingly, the use of platinum is normally preferred for the bottom electrode when a ferroelectric thin film is used as an actuator for an ink jet recording head.
Platinum is therefore favorable for the bottom electrode of a electromechanical transducer, but there has been a need for a bottom electrode having a structure capable of preventing the admixture of impurities (such as titanium) into the ferroelectric thin film during the electromechanical transducer manufacturing process, and of enhancing the adhesion between the bottom electrode and the installation surface and between the bottom electrode and the ferroelectric thin film.
When an alloy of platinum and iridium was used for the bottom electrode of an electromechanical transducer as disclosed in Japanese Patent Laid-Open No. 07-245236, or when iridium oxide was used as disclosed in Japanese Patent Laid-Open No. 07-245237, a problem was encountered in that residual stress was generated in the bottom electrode containing iridium in the course of baking and crystallizing the ferroelectric thin film, and this residual stress diminished the characteristics of the electromechanical transducer. For example, the residual stress generated in a bottom electrode could impart strain to the ferroelectric thin film and undesirably lower the percentage of volumetric change.
In view of this, it is an object of the present invention to provide a method for manufacturing a ferroelectric thin film device with which the crystal orientation of a ferroelectric thin film can be controlled as dictated by the intended application of an electromechanical transducer. It is a further object to provide an ink jet recording head whose ink discharge drive source is an electromechanical transducer obtained by this manufacturing method, as well as a method for manufacturing this head, and an ink jet printer that makes use of the same. Another object of the present invention is to provide a nonvolatile ferroelectric memory device in which a ferroelectric thin film device obtained by this manufacturing method serves as a capacitor, and a method for manufacturing this memory device.
Yet another object of the present invention is to provide a method for manufacturing an electromechanical transducer having very reliable drive characteristics when iridium alone is used as the material for the bottom electrode.
Yet another object of the present invention is to provide an electromechanical transducer with which adhesion with the installation surface can be enhanced without diminishing the piezoelectric characteristics, and an ink jet recording head and an ink jet printer that make use of this transducer. Another object of the present invention is to provide a method for manufacturing an electromechanical transducer having a layer structure with which adhesion with the installation surface can be maintained without diminishing the piezoelectric characteristics.
Still another object of the present invention is to provide an electromechanical transducer having good piezoelectric characteristics as a result of reduced residual stress during baking, an ink jet recording head and printer that make use of this electromechanical transducer, and a method for manufacturing an electromechanical transducer.